1. Field of the Invention
The present invention relates, in general, to an artificial interfusion implant installed in an intervertebral space created from the removal of a damaged spinal disk, and more particularly, to an expandable interfusion cage, expandable to increase in diameter.
2. Description of the Prior Art
In order to stabilize, for extended periods, fractures of a damaged spinal disk in humans, a fusion can be implemented, in which at least two bones of the spinal column are connected with each other by a bony bridge. In implementing interbody fusion as well known to those skilled in the art, so as to restore a normal spatial relationship, a spinal disk is partially cut, and a bone segment is positioned between adjacent vertebrae in a space already occupied by a disk material, to provide immediate stability through mechanical support. Also, by subsequent permanent cross-conjugation of vertebral bones, long-term stability is provided.
The main application of the invention is to provide implants designed to be slid or inserted from a posterior direction between the confronting faces of two consecutive vertebrae in order to maintain a predetermined distance between them and to restore stability to the spinal column, e.g. after a failure of the corresponding joint, by fixing the two vertebrae together.
Several techniques are known at present for restoring a normal lumbar lordosis in this way, by implanting either a graft which in time fuses the vertebrae together, or a prosthesis which fixes them together immediately, while maintaining the possibility of achieving a fusion between the vertebrae in times.
In the second above-mentioned technique, use is made mainly of implants, also known as “cages”, some of which are hollow, rigid, and contain only one piece, with inside/outside intercommunication slots for receiving a bone graft which, via said slots, subsequently fuses with the adjacent vertebrae on either side. In this field, reference can be made to International Patent Publication No. WO 96/08205 published on Mar. 21, 1996 for an “Intervertebral fusion cage of conical shape”, and European Patent Publication No. EP 637 440 published on Feb. 8, 1995 for an “Intersomatic implant for the spinal column”. Nevertheless, cages of those types are of outside dimensions that are given and fixed, whereas the distances between pairs of vertebrae are not constant. In addition, the inclinations of the facing vertebral faces to which a given angular position is to be imparted do not enable rigid cages to be used from a posterior direction. That is, they can be inserted only from an anterior direction.
As a result, other types of cages have been developed with two substantially parallel branches connected to a rigid body through which it is possible to turn a worm screw system which then moves a wedge in a screw engagement on a worm screw from an initial position close to the distal ends of the branches towards the body linking the branches together, thereby splaying the two branches apart angularly. It is then possible to insert such a cage, in an initially flat shape, between the vertebrae, and then by turning the drive axis of the wedge, the desired angle between the branches is adjusted or set from a posterior access.
Such cages or implants are described, for example, in European Patent Publication No. EP 664 994 published on Aug. 2, 1995, entitled “Vertebral intersomatic cage” or in France Patent Publication No. 2 719 763 published on Nov. 17, 1995, entitled “Vertebral implant”.
However, such devices which are more mechanical than hollow and rigid cages, and therefore more complex, leave a smaller inside volume for the fusion graft, and because of their flat shape which is not circularly symmetrical, even though they are better at ensuring a given bearing angle between the vertebrae, they require a passage of the same rectangular section to be prepared to receive them, which complicates implementation.
The problem posed is thus to be able to have implants or cages available making it possible simultaneously to ally the shape of a conventional rigid cage, firstly to facilitate implantation and secondly to provide a larger inside volume, with the possibility of increasing the diameter of the distal end of the cage to a given value relative to its end situated adjacent to its point of surgical insertion, after it has been put into place, and corresponding to the posterior face of the vertebrae, while having as few mechanical elements as possible.
In an effort to address this problem, an expandable osteosynthesis cage is disclosed in International Publication No. WO 1998/10722 dated Mar. 19, 1998. In this regard, referring to FIGS. 1 and 2, the cage 1 has a hollow shape and includes a seat 7 and branches 5. The seat 7 serves as a cylindrical body and is pierced by an orifice 8. Each of the branches 5 is connected at one end thereof to the seat 7. A spacer 2 is movably assembled in an inside volume 9, that is, an inside space, of the cage 1. As the spacer 2 is moved toward the distal ends of the branches 5, the branches 5 are biased radially outward by the spacer 2 and thereby expanded.
The expandable osteosynthesis cage partially solves the defects occurring in the conventional art, but some of the defects still remain. In particular, while the spacer 2 is movably assembled in the inside space having a tapered section, if the spacer 2 is moved to the distal ends of the branches 5 to bias radially outward the branches 5, positioning of the spacer 2 at the distal ends of the branches 5 is structurally unstable. Therefore, after the installation of the expandable osteosynthesis cage is completed, the spacer 2 is likely to be unintentionally pushed inward into the inside volume 9 of the cage 1. Further, as can be readily seen from FIG. 2, because an inner surface of each branch 5 is tapered so that a thickness of each branch 5 is increased toward the distal end thereof, when the spacer 2 is moved to the distal ends of the branches 5 to expand them radially outward, limitations exist in increasing the expansion of the branches 5.